How to render out in VSE only a region of the frame?
It depends on the blender version that you use. The transform tools are heavily changed in version 2.92 (currently in beta). The changes are described at https://wiki.blender.org/wiki/Reference/Release_Notes/2.92/More_Features and more in depth at https://developer.blender.org/rBe1665c3d31.
Before version 2.92 the use of the crop tool was problematic. Now, it works as expected and you can crop in pixel values from top,bottom, left and right.